1. Field of the Invention
The present invention relates to a digital TV system, and more particularly, to an apparatus and a method of transcoding image data in a digital TV system. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for converting a particular data bit rate of Moving Picture Experts Group (MPEG) data bit streams into a different data bit rate.
2. Discussion of the Related Art
Recently, encodements such as MPEG and many others are widely used in order to reduce the amount of digital video or audio data for storing and transmitting. Particularly, since various application processes such as data searching, Picture-In-Picture (PIP), data merging, data editing, and transmission rate conversion are necessary, an image data transcoding device and/or method for converting a particular data bit rate of an MPEG bit stream to a different data bit rate is required. Some of the cases where such transcoding device or method is necessary are conversion from an MPEG bit stream into a JPEG bit stream, conversion from Digital Video (DV) data of a digital camcoder into MPEG bit streams, or conversion from a High Definition (HD) MPEG bit stream into a Standard Definition (SD) MPEG bit stream.
In general, a typical image transcoding device includes a encoding part and a decoding part. When a set of image data having a particular data bit rate is converted to a set of data having a lower data bit rate in the image transcoding device, the image quality of the data is generally degraded. For this reason, a method of eliminating the high frequency AC coefficient in the MPEG decoder, a method of converting the data bit rate by re-quantization in the MPEG decoder, or a method of using a cascaded transcoder connecting the MPEG encoder and decoder is often used to maintain the image quality when converting the data bit rate of a set of image data.
First, the method of reducing the bit rate by eliminating the high frequency AC coefficient eliminates the DCT coefficient of each region having a data bit rate higher than the target bit rate, requiring a very simple hardware to achieve the objective. However, the image quality is still not good enough because an accumulative error occurs when eliminating each DCT coefficient.
Next, the method of the re-quantizing results a reasonably good image quality, but it requires a very complicated hardware. In addition, the third method requires a very complicated hardware since the cascaded transcoder includes an MPEG-2 encoder. However, this method achieves the best image quality among the mentioned methods.
Since the data storage capacities of many digital image related systems such as a digital VCR system are often limited, they do not store all of the MPEG bit streams received. For example, the data bit rate of the data streams must be reduced by half in order to increase the data storing speed two times. For the personal digital system, a method of reducing the data bit rate of a set of image data using a simple hardware is preferred due to its low cost. Therefore, the method of eliminating the high frequency AC coefficient or re-quantizing is more frequently used. The method of using the cascaded transcoder results a good image quality and can be used for a Video On Demand (VOD) server or a broadcasting station, but it further includes complicated processes including determining a new macro block mode or motion compensation mode.